Chapter 10: Wind—Small Scale and Local SystemsI)Scales of motion: hierarchy of motion arranged according to their sizeA)Microscale: diameters of few meters or less1)Eddies: spinning globs of air that have a life history of their own2)Disperse smoke, sway branches, swirl dust3)Form by convection or by the wind blowing past obstructions4)Short-lived, lasting only a few minutesB)Mesoscale: winds range from a few kilometers to hundreds of kilometers in diameter1)Last minutes, hours, or a day2)Include local winds, t-storms, tornadoes, and small tropical stormsC)Synoptic scale: water map scale1)Dominate regions of hundreds to thousands of square kilometers2)Include high and low pressure areas3)Varies in length from days to weeksD)Macroscale1)Planetary (global) scale2)largest wind patternsII)On microscopic level friction arises as atoms and molecules of two surfaces seem to adhere then snap apart as they slide across one anotherA)Viscosity1)friction of fluid flow2)Due to exchange of air molecules moving at different speeds3)When slowing of a fluid is due to random motion of the gas molecules the viscosity is referred to as molecular viscosityB)Laminar flow: mass or air gliding horizontally and smoothly over a stationary mass or air1)At boundary separating the air layers there is constant exchange of molecules b/t stationary air and flowing air2)Molecular exchange slows down the moving airC)Eddy viscosity: internal friction produced by turbulent whirling eddies1)Near surface, it is related to the roughness of the ground2)As wind blows over a landscape it breaks into a series of irregular, twisting eddies that can influence the air flow for hundreds of meters above the surface3)Within each eddy wind speed and direction fluctuate rapidly producing wind gusts (irregular air motion)D)Mechanical turbulence: eddy motion created by obstructions1)Creates a drag on the flow of air2)Drag is greater than that caused by molecular viscosityE)Frictional drag is greatest closest to earth’s surface, as we move away from earth’s surface wind speeds tend to increase with height above the groundF)Planetary boundary level: atmospheric layer near the surface that is influenced by friction 1)Top usually near 1000m2)Frictional layer may be higher with:a)strong winds: creates strong mechanical turbulenceb)rough terrain: creates strong mechanical turbulencec)Surface heating: produces steep lapse rate and strong thermal turbulencei)thermals rise and convection cells formii)Thermal turbulence: resulting vertical motioniii)At minimum in early morningiv)Increases the intensity of surface heating and degree of atmospheric instability

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